Resistance heating element for vacuum furnaces and the like



Feb. 7, 1961 Filed Nov. 26, 1957 H. W. RESISTANCE HEATIN WESTEREN GELEMENT FOR VACUUM FURNACES AND THE LIKE 2 Sheets-Sheet 1 Feb. 7, 1961H. w. wEs'rERr-:N 2,971,039

RESISTANCE HEATING ELEMENT FOR VACUUM FURNAQES AND THE LIKE 2Sheets-Sheet 2 Filed Nov. 26, 1957 United States Patent RESISTANCEHEATING ELEMENT FOR VACUUM F URNACES AND THE LIKE Herbert W. Westeren,Barrington, RJ., assignor to C. I. Hayes, Inc., a corporation of RhodeIsland Filed Nov. 26, 1957, Ser. No. 699,015

3 Claims. (Cl. 13-25) The instant invention relates generally to heattreatment furnaces and, more particularly, to heat treatment furnaces ofthe high-vacuum type.

A primary object of the instant invention is the provision of a noveland improved resistance heating element specifically designed for useand operation in the so-called cold-wall type of vacuum furnace whereinthe heating element is positioned within the high-vacuum chamber.

Another important object of my invention is the provision of aresistance heating element of the character described which willeffectively operate on a relatively low electrical potential.

Another object is the provision of a resistance heating element having amaximum heat transmitting surface and so constructed as to provide rapidand uniform heatlng.

A further object of my invention is the provision of a novel andimproved resistance heating element for vacuum furnaces of the cold-walltype, which heating element enables a complete and effective reliectiveshield to be employed.

Still another object of the instant invention is the provision of aresistance heating element of the character dcscribed having a completeand effective reflective shield, the latter of which may be easilyremoved from the furnace for cleaning and the like.

A further object is the provision of a resistance heating element whichin itself is readily removable from the furnace without the necessity ofdisrupting the terminal lead connections which are employed for feedingcurrent to the unit.

It is also an object of my invention to provide a resistance heatingelement which is of rugged and durable construction so as to beself-supporting, whereby to eliminate the necessity of using anyrefractory or other electrical insulation material within the vacuumchamber.

Other objects, features and advantages of the invention will becomeapparent as the description thereof proceeds when considered inconnection with the accompanying illustrative drawings.

In the drawings which illustrate the best mode presently contemplated byme for carying out my invention:

Fig. l is an elevational view, in section, of a highvacuum cold-wallfurnace embodying the instant invention;

Fig. 2 is a section taken on line 2-2 of Fig. 1;

Fig. 3 is an elevational section, on an enlarged scale,

`of the terminal connection which forms a part of the instant invention;

Figt 4 is a section taken on line 4-4 of Fig. 3; and

j Fig. 5 is a perspective detail, on an enlarged scale, of

the resistance heating element per se.

It has been found desirable to provide a novel and improved resistanceheating element for a high-vacuum heat treatment furnace of thecold-wall type. Since heat transfer in a vacuum is accomplished byradiation and since the rate of heat transfer is directly affected bythe area of heat transmitting surface, it has rst of all been founddesirable to provide a resistance heating element having a maximumradiating surface. In addition, since it is well known that the presenceof refractory within a vacuum chamber makes effective vacuumizing of thechamber more difficult, and since refractory or any other electricalinsulationmay become a conductor due to deposits or contaminationthereon resulting from vaporization of portions of the work load beingheat treated in a high vacuum, the instant invention provides aresistance heating element so constructed as to be self-supportingwhereby to eliminate the necessity of utilizing refractory or otherelectrical insulation within the vacuum chamber. in addition, therelatively heavy construction of the heating elements hereinafter to bedescribed enables the desired heat to be obtained by application of arelatively low electrical potential since the heating elements present alow electrical resistance to the electric current being suppliedthereto. The use of a low electrical potential greatly reduces thelikelihood of undesirable ionization occurring within the vacuumchamber.

ln addition to the above features, the instant invention enables a morecomplete and effective reflective shield to be utilized, it beingapparent that the efficiency of the entire furnace is closely tied inwith the effectiveness of the heat baiiles which are employed. Also, aswill hereinafter become apparent, the instant arrangement enables boththe heating element and the reective shield to be readily removed fromthe furnace for cleaning or repairs with a minimum of difficulty.

Referring now to the drawings, and more particularly to Fig. l thereof,there is shown generally at 1G a highvacuum heat treatment furnace ofthe cold-wall type. The furnace 1li comprises a pressure vessel 12having an upper section 14 and a lower section 16. It will be understoodthat pressure vessel 12 is constructed so as to withstand atmosphericpressure with a minimum of leakage while operating at a high vacuum, andpreferably, the said vessel is constructed of nickel-clad steel. As willbe noted, the upper section 14 and the lower section 16 are maintainedin tight engagement as at 18, by any desirable means, to provide avacuum tight seal; and in order to further insure the effectiveness ofthis seal, an O ring 20 is provided intermediate the two sections. Uppersection 14 actually defines a cooling chamber 2-2, it being noted thatthe said cooling chamber is of reduced diameter with respect to lowersection 16, the latter of which defines a heating chamber 24.

Hydraulic lifting mechanism 26 is secured to the furnace upper section14 and is adapted to raise the said upper section from its engagementwith lower section 16. Once so raised, the upper section 14 is adaptedto be swung away from the axis of the lower section by means of apivotal mounting (not shown) in order that the furnace may be loaded ina manner hereinafter to be more clearly described. Supporting braces orstandards 28 are secured to lower section 16 for mounting the furnace 10in its normal, upright position.

As will be clearly seen, both the upper section 14 and the lower section16v are substantially completely enclosed by a water jacket 30 in orderto maintain the outer furnace wall cool for safer handling of theapparatus and for more rapid cooling of the heating chamber whendesired. lt will be noted that the water jacket 3i) does not cover anyof the welds in the vacuum chamber wall, the reason for this being thatif a leak should develop at one of these welds, water would immediatelyow into the furnace if the water jacket did cover the said welds. Also,the absence of a water jacket over the welds enables any leaks whichar'e found to exist to be more easily repaired.

A conventional vacuum pumping port 32 is provided adjacent the upperextremity of lower section 16, while 3. a sight port 34 is providedadjacent the lower extremity of section 14. It will be understood thatsuction is applied to the port 32 when it is desired to evacuate thevessel 12,.while^port.24v-simply provides aviewing aperture throughwhich the interior of lower section 16 may be readily seen from outsidethe furnace.

Extending upwardly from they bottom of lower section 16 and centrallypositioned within the heatingr chamber 24 is an elongated thermocouple36, it being understood that the said thermocouple extend through thefurnace wall by means of a vacuum-tight connector 38 mounted on abracket 40 carried by the framework 28. Telescopingly mounted overthermocouple 36 is an elongated, hollow shaft 42 having an enlarged baseportion 44 which when in its lowermost position, as illustrated in Fig.1, covers the connector 38, and which is maintained in its centrallydisposed position by means of positioning lugs 46. As will be noted, rod42 carries a work tray or pedestal 48, the upper surface 50 of which ispreferably of reec'tive material whereby to maintain the heatcentralized within the chamber 24 as much as possible. An additionalradiation shield or reflective bafe 52 is mounted just below pedestal 48on the rod 42, and still another shield 53 spans the bottom of chamber24 whereby to provide additional heat baffles. Likewise, an upper baftie54 is slidingly mounted on rod 42 adjacent its upper extremity wherebyto prevent the heat from travelling upwardly into cooling chamber 22.

Rod 42, at its upper extremity 56, is secured to a cable 58, which cableis adapted to be taken up by a handdriven winch 6d located in the upperportion of cooling chamber 22 and extending through a rotary vacuum seal62 in the wall thereof for exterior operation as at 64. Thus, when winch60 is operated, rod 42 may be raised into cooling chamber 22 carryingwith its pedestal 48, baffle 52, and any work load or basket (not shown)which may have been deposited on the surface 50. As will be apparent,the thermocouple 36 will function as aligning means for maintaining thework load properly centered during its ascent, while at the same timethe thermocoupie enables the temperature of the heating chamber to bereadily ascertained.

The heating unit which forms a salient part of the instant inventioncomprises three identical grid members 66, each constructed of an alloywhich is substantially 50 perecnt nickel and 50 percent iron. As will beseen most clearly in Figs. 2 and 5, the grid members are assembled insubstantially cyindrical form and are connected at one end by an ironconductor ring 68 welded to the grid ends so as to form a goodelectrical connection. At their opposite ends, each grid 66 is providedwith an iron segment 7i?, each of said segments having secured thereto,as by welding, an iron terminal member 72. Each of the terminals 72 isprovided with a mounting assembly generally designated at 74, note Fig.3. The assembly 74 comprises an outer cylinder 76 extending through thebottom wall of lower section 16 and secured thereto by a vacuum-tightweld as at 78. An outwardly extending flange 80 is secured to saidcylinder by welding or the like and is provided with threaded apertures82. An inner cylinder 84 having a conical upper seat 86 is adapted to bemounted within the cylinder 76 in spaced relation thereto. Morespecifically, the cylinder 84 is provided with an outwardly extendingflange 88, which flange is provided with an opening 98 therethrough,said opening 90 being in substantial alignment with the afore-describedthreaded opening 82 in the flange 80. An insulating gasket 92 ispositioned between the flanges 80 and 88, and bolts 94 of electricalinsulating material are then utilized to clamp the said flanges 88 and88 together as shown most clearly in Fig. 3. As will be obvious, thecylinders 76 and 84 are completely insulated from each other from anelectrical standpoint, first of all, because their flanges are separatedfrom each other by an insulating gaskety 92 and are interconnected byinsulated bolts 94, and' secondly, because the walls of said cylindersare spaced from each other as at 96.

A threaded opening 9S is provided in the lower extremity of eachterminal '72 for receiving the threaded extension of hollow shoulderbolts 102. As will be noted, the threaded extension 10i) extends througha specially provided opening 104 in the upper portion of cylinder 84,and lupon being tightly threaded within the opening 98, the uppershoulder 106 of the bolt 182 is tightly drawn against surface 188 of thecylinder 84 to provide a vacuum seal therewith. To further insure theeffectiveness of this seal, an 0 ring 11@ is positioned between thesurfaces 186 and 188. Vent passages 112 and 114 are provided incommunication with the openings 98 and 104, respectively, in order toenable any air which may be trapped in these openings to be readilyevacuated when suction is applied to port 32 for vacuumizing thepressure vessel.

rthe bolt 102 is preferably water cooled by means of a plug 116 which isadapted to be threaded into the open end of bore 118, the said plugcarrying an upwardly extending open-ended plastic tube 120. As will benoted, plug 116 threadedly receives a tube 122, which tube is incommunication with the interior of tube 12@ by means of opening 124provided in the plug 116. A second tube 126 is threadedly received inthe wall of bolt 162 and is in communication with bore 118 whereupon acooling liquid may be continuously circulated through the interior ofthe bolt 102, it being immaterial whether the fluid enters through tube122 or 1.26. Heavy block terminals (not shown) are secured to the lowerportion of the cylinders 84 as at 128 whereby electrical current beingfed through the said terminals will pass upwardly through cylinder 84 toterminal '72 and thence to segment 7i) and grid 66. In order to insuregood electrical contact between terminal 72 and cylinder 84, the matingconical surfaces of these elements are preferably silver plated.

As will be obvious, the attore-described terminal arrangement andassembly enables an eletric current to be fed to the grids 66 while atthe same time maintaining complete insulation with the chamber per se.Since the entire unit is insulated from ground, and since the terminals,segments and grids are spaced from any interior components, such asbafes, etc, there is little likelihood of a short circuit developing dueto multiple grounding. This means that no refractory or other electricalinsulation is necessary within the heating chamber, a highly desirablefeature since refractory is not only difficult to effectively vacuumize,but also when working in high vacuumsl and at high temperatures it ispossible that deposits or contamination which are likely to result fromvaporiz ation of portions of the work load may cause the insulator tobecome a conductor hence making short circuiting likely'.

In addition to the foregoing, the above-described terminal arrangementisl further advantageous in that it enables the heating unit to beremoved as a unit without the necessity of separating and removing theheavy terminal blocks from their connections to the cylinders 84. Inother words, whenever it is desired to remove the heating unit, it issimply necessary to unscrew the shoulder bolt 102 whereupon the heatingelements with their terminals 72 are free for removal as a unit, therebeing no necessity whatsoever for disrupting the terminal connection tothe cylinders 84.

The heating unit has been specifically designed to provide high poweroutput at low voltage, which, as afore-described, is advantageous inhigh vacuum applications since short circuits can be effected whenworking at high voltages due to ionization. Since high power output ismore economically utilized by a three-phase system the grid 66 have beenarranged in cylindrical form so as to provide three equal resistanceswith one end of the cylindrically' formed grids connected together bythe conductor ring' 68 and with the opposite ends thereof provided withterminals 72 rigidly secured to the grid ends by means of the segments7i). As will be apparent, there is one terminal for each phase, andhence, when an A.C. three-phase electrical current is fed to the saidterminals, the grids, segments and ring will heat to approximately thesame temperature thereby insuring relatively uniform heating throughoutthe overall unit. This uniformity of heating is achieved due to the factthat the electrical resistance throughout every portion of each of thegrids is uniform, and since the grids, segments and ring are constructedof relatively heavy material, they afford a low electrical resistance tothe electric current passing therethrough, thereby enabling the desiredheat to be Generated at relatively low voltages. In addition, therigidity and ruggedness of the heating unit enables it to beself-supporting, which, of course, means that no refractory or otherelectrical insulation material need be used to support the unit. Ashereinbefore indicated, this is a highly desirable feature of theinstant arrangement.

As will be seen most clearly in Fig. l, the grids 66 closely surroundany work that is supported on the pedestal 4S and are spaced from saidpedestal sufliciently for the latter to freely move upwardly when raisedby the winch 6l?. As will be apparent, the cylindrical arrangement ofthe flat sheet-like grids affords a maximum heat transfer surface, andin addition, this particular arrangement enables a complete andeffective reflective shield or baille 13@ to surround the heating unit.As will be obvious, baille 125i) is nothing more than a straightcylinder having a highly reflective surface, which is freely mounted onhorizontal baille 53, it being understood that notches 132 are providedfor clearing the terminals 72 so that the said terminals and shield 130do not come in contact with each other. As will be apparent, baille 13)will function in cooperation with the afore-described reflectivesurfaces 50, 52, 53 and 54, to effectively and tightly maintain the heatgenerated in the grids 66 within the treatment area. In addition, theballles may easily be removed for cleaning or repairs Whenever desired.

To load the furnace 10, the pedestal 48 is raised to the upper section14 after which the latter is raised from lower section 16 by means ofthe hydraulic lifting mechanism 26. Once so raised, upper section 14 ispivotally swung away from the axis of the lower section, and thepedestal 4S is then lowered. The work tray or work load is then placelon surface Sil of the pedestal 48, and the pedestal is then once againraised so that the work is completely encased within chamber 22. Theupper section is then swung baclc into position, lowered into engagementwith the lower section, and the work is then lowered so as to bepositioned within the heating unit as per the position of the partsillustrated in Fig. l. Once the work has been heat treated, the pedestal48 is again raised into chamber 22, which chamber functions as a coolingarea. It will be noted that when the pedestal 48 has been raised intochamber 22, the lower baille 52 will function to prevent any heat whichpasses upwardly from the grid 66 from entering the said chamber 22.

Thus, it will be seen that there has been provided in accordance withthe instant invention a high-vacuum heat treatment furnace having ahighly novel and greatly improved heating element. The instantarrangement enables a high degree of heat to be obtained at a relativelylow electrical potential and further results in a highly uniform heatingarrangement. ln addition, the heating element may be more effectivelybailled, which is a highly important feature. Also, since the heatingunit is selfsupporting no refractory or other electrical insulation needbe employed within the vacuum chamber.

While there is shown and described herein certain specific structureembodying the invention, it will be manifest to those skilled in the artthat various modifications and rearrangements of the parts may be madewithout departing from the spirit and scope of the underlyino inventiveconcept and that the same is not limited to the particular forms hereinshown and described except insofar as indicated by the scope of theappended claims.

l claim:

l. A resistance heating element for vacuum furnaces and 'the like,comprising at least three elongated grid members that are spaced fromeach other to define a cylindrically arranged structure, a conductorelement interconnecting one end of said grid members, said grid membersthereby being insulated from each other throughout the length thereofexcept at their interconnected ends, and means operatively engaging eachof said grid members at the insulated end thereof for feeding electriccurrent thereto, said electric current feeding means including anelectrically conductive segment bar secured to the insulated end of eachgrid member and extending the width thereof, the cross section of saidsegment bars being greater than the cross section of said grid members,wherein the electrical resistance of said segment bars is lesser thanthat of said grid members, so that current introduced into said segmentbars will flow evenly therethrough to provide an even distribution ofcurrent throughout the length of said grid members, a terminal membersecured to each segment bar, and a current carrying element engagingeach of said terminal members.

2. A resistance heating element for vacuum furnaces and the like,comprising a plurality of elongated grid members that are spaced fromeach other to define an open-ended cylindrically arranged structure,means interconnecting one end of said grid members and defining aconductor element, said grid members thereby being insulated from eachother throughout the length thereof except at their interconnected ends,each of said grid members including a segment bar joined to theinsulated end thereof, said segment bars being constructed and arrangedto transfer an even distribution of current throughout the length ofsaid grid members, and means operatively connected to said segment barsfor feeding electric current thereto.

3. A resistance heating element for vacuum furnaces and the like,comprising a plurality of elongated arcuate shaped grid members that arecircumferentially spaced from each other to deiine an open-endedcylindrically arranged structure, said grid members being interconnectedat one end by a conductor ring and being insulated from each otherexcept at their interconnected ends, each of said grid members having asegment bar joined to the insulated end thereof, the cross section ofsaid segment bars being greater than the cross section of said gridmembers wherein the electrical resistance of said segment bars is lessthan that of said grid members, so that current introduced into saidsegment bars will flow evenly therethrough to provide an evendistribution of current throughout the length of said grid members, andmeans connected to each of said segment bars for feeding electriccurrent thereto.

References Cited in the tile of this patent UNITED STATES PATENTS715,505 Potter Dec. 9, 1902 1,496,299 Clifford June 3, 1924 2,337,679Osterberg Dec. 28, 1943

